UX improvements

[DPS2004]

• Adds a Settings menu accessible from the pause screen.
• Adds more options for the main menu, including layout and color
• Adds 3D Banner to 3DS build
• Adds support for built in carts (referred to as pack-ins) on 3DS and Windows. Disabled by default in the Makefile
• Adds makefile for SDL2 on Windows (for development purposes)

[DPS2004]

Hi, thank you for the detailed response.

• Splitting this up into smaller commits will be hard, because the options cart and the menu cart are somewhat built on top of each other. In the future, though, I will try to be more careful about not shoving things into one branch during development.
• Since the Pico-8 logo can't be used for the banner and the no label image, I think it is safe to assume that the sprites from Jelpi should not be used either in the banner. So for now, I will revert back to the 2D banner.
• I will change the default value for the resize hotkey to be enabled, and I will try to add a loading indicator to the fancy menu.
• carts.zip being included was not on purpose, and I chose to use a header instead of romfs (or equivalent for other platforms) because that would require a lot of platform specific code.
• the cart unzipping only once thing is a pretty easy fix, same with the classic menu artifact.

I will try to get these changes done over the weekend.

[jtothebell]

Sounds good, thanks. No rush, of course, I appreciate your work. This also made me think I should probably put together PR and other community guidelines. Unfortunately, since you're the first significant feature contributor, I'm figuring that out as I go.

[DPS2004]

(closing for now while i reorganize into multiple PRs)